1. Field of the Invention
The present invention relates to a vehicle suspension system for a steerable wheel. More particularly, it relates to a vehicle suspension system for a steerable wheel which will enhance a free layout degree by making it easy to provide an imaginary kingpin axis and imaginary links, and to reduce the change ratio of a height of a roll center, and as a result, can improve all sorts of safety such as straight ahead safety by reducing responsiveness with respect to the steering operation.
2. Description of Related Art
In general, a vehicle suspension system for a steerable wheel connects a shaft and a frame to each other, and absorbs vibrations and impacts from a road surface while the vehicle is running, and so, a safer vehicle and improved ride comfort can be obtained.
To absorb impacts, a vehicle suspension system should be connected flexibly in a vertical direction, and to endure driving force, braking force and centrifugal force generated at a wheel during the vehicle's turning, a vehicle suspension should be connected firmly in a horizontal direction.
The vehicle suspension system is classified structurally as an axle integral suspension system and an independent suspension system. While an axle integral suspension system is applied to a large vehicle like buses, trucks and rear axles of passenger cars, an independent suspension system is applied mainly to an axle of a passenger car. To improve ride comfort and running safety, an independent suspension is bisected to act independently.
Among these suspension systems, the present invention relates to an independent suspension system.
There are many kinds of independent suspension systems. As a suspension system applied to a steerable wheel, Macpherson type and Wishbone type are applied widely.
A vehicle suspension system for a steerable wheel is designed to control a position of a wheel with respect to the vehicle body to obtain a better positioning in a vehicle operation as well as absorb vibrations or impacts. The axle is thereby prevented from directly transmitting an impact or vibration from a road surface to the vehicle body, so that optimal handling safety and running safety can be obtained.
The front wheel is mounted to be changeable in its direction from right to left or from left to right, centering the kingpin, and is also mounted with a geometrical angle to meet the front suspension requisites.
Mounting the front wheel with the geometrical angle is called wheel alignment. The wheel alignment is determined by several elements, but the optimal operation of the front wheel is achieved by the supplemental operation of the elements with respect to each other.
A kingpin inclination among the elements reduces the handling force of a steering wheel with a camber, absorbs any impact generated from the vehicle motion and then increases restitution force of the steering wheel. The turning safety of the straight ahead position of the vehicle and the handling safety in a vehicle's turning position are thereby obtained.
The kingpin inclination is such that the kingpin is set up with its central line lying at a predetermined angle with respect to the vertical line when the vehicle is viewed from the front.
An interval between the central line of the kingpin and a central line of the wheel is called an offset, and as the offset gets smaller, the responsiveness to a steering operation becomes reduced to improve the handling safety during the vehicle's turning or braking.
Vibrations have something to do with the vehicle safety. Rolling, pitching, yawing as well as bounce occur in a vehicle during the vehicle's driving. Only if these vibrations are absorbed softly, can ride comfort and safety be improved.
As the vibration of a vehicle is concerned, vibration generated when the vehicle is turning is defined as rolling, which means that a vehicle vibrates in the left-and-right directions, and rolling has an important effect upon cornering safety.
A certain basic point where rolling occurs is called a roll center (RC). Rolling occurs because the centroid of a vehicle is higher than a roll center. Accordingly, the greater the change in the height of a roll center, the more the gravity of a vehicle moves. Therefore, this phenomenon results in a greater slip angle, so the running safety and the handling safety get worse.
To ensure running safety, the variation ratio of the height of RC is desired to be minimized.
Considering the above, the conventional suspension system is described in detail hereinafter.
FIG. 5A illustrates a general Macpherson type suspension system, including a strut arm 53 formed integrally to a steering knuckle 51, provided with a shock absorber 52 inward, a spring 54, a control arm 55, and a ball joint 56.
This suspension system has some advantages of simple structure, a small spring, light mass, and a small effective volume or an engine room.
But in order to make the kingpin offset (.alpha.) reduced or negative (-), an upper supporting point 57 of the strut arm 53 should be displaced toward the engine room, or the wheel side end of the control arm 55 connected to the ball joint 56 should be displaced outside.
However, if the upper supporting point 57 of the strut arm 53 is displaced toward the engine room, the effective volume of an engine room is reduced and the kingpin angle (.alpha.) becomes excessively large only to have a bad effect on cornering safety of a vehicle.
If the wheel side end of the control arm 55 connected to the ball joint 56 is moved outside, it is actually impossible to reduce the kingpin offset (.alpha.) because it will interfere with a brake disk attached to a wheel 58.
The change of a camber angle and tread is illustrated by a dotted line in FIGS. 4A and 4B, so the running safety and the cornering safety get worse.
FIG. 5B illustrates a general Wishbone type suspension system, including upper and lower control arms 61, 62, a steering knuckle 63, a spring assembly 65 including a shock absorber 64 and ball joints 66, 67 connecting the upper and lower control arms 61, 62 to the steering knuckle 63.
To adjust the kingpin offset (.alpha.), a vehicle body side connecting point 68 of the upper control arm 61 should be displaced toward the engine room, or the ball joint 67 of the lower control arm 62 should be displaced outside.
But, in a Wishbone type suspension system, as well as Macpherson type suspension system, if the vehicle body side connecting point 68 of the upper control arm 61 is displaced toward the engine room, the effective volume of an engine room is reduced and the kingpin angle (.alpha.) becomes excessively large to adversely affect cornering safety of a vehicle, and if the ball joint 67 of the lower control arm 62 is displaced outside, it is actually impossible to reduce the kingpin offset (.alpha.) because it will interfere with a brake disk attached to the wheel 69.